This invention is related to the field of bacterial genetics and more particularly to the field of respiratory pathogens of farm animals.
P. haemolytica as a pathogen causes serious economic damage to the animal farming industry. Vaccines which have been developed in an effort to control the disease have met with variable but limited success. Because the disease is caused in significant part by the animals"" own reaction to P. haemolytica infection, inappropriately designed vaccines may actually worsen the clinical condition of infected vaccinates. Thus, there is a continuing need in the art for safe and effective vaccines which can reduce the morbidity and/or mortality of ruminants due to P. haemolytica. 
It is an object of the present invention to provide a P. haemolytica bacterium useful as a vaccine strain.
It is another object of the present invention to provide a method of inducing immunity to pneumonic pasteurellosis in ruminants.
It is an object of the present invention to provide a vaccine strain against pneumonic pasteurellosis.
Another object of the invention is to provide a ruminant feed.
Another object of the invention is to provide a temperature sensitive plasmid for manipulation of P. haemolytica. 
These and other objects of the invention are achieved by one or more of the embodiments described below. One embodiment of the invention provides a P. haemolytica bacterium which expresses no biologically active leukotoxin, expresses a form of leukotoxin molecule which induces antibodies which specifically bind to leukotoxin, and contains no foreign DNA.
Another embodiment of the invention provides a method of inducing immunity to pneumonic pasteurellosis in ruminants. A bacterium is administered to a ruminant. Immunity to the bacterium is thereby induced. The bacterium expresses no biologically active leukotoxin, expresses a form of leukotoxin molecule which induces antibodies which specifically bind to leukotoxin, and contains no foreign DNA.
Yet another embodiment of the invention provides a feed for ruminants. The feed comprises a bacterium which expresses no biologically active leukotoxin, expresses a form of leukotoxin molecule which induces antibodies which specifically bind to leukotoxin, and contains no foreign DNA.
Even another embodiment of the invention provides a vaccine for reducing morbidity in ruminants. The vaccine comprises a P. haemolytica bacterium which expresses no biologically active leukotoxin, expresses a form of leukotoxin molecule which induces antibodies which specifically bind to leukotoxin, and contains no foreign DNA.
Still another embodiment of the invention provides a temperature sensitive plasmid. The plasmid replicates at 30xc2x0 C. but not at 40xc2x0 C. in P. haemolytica. Moreover, it is of the same incompatibility group as the plasmid which has been deposited at the ATCC with Accession No. 98895.
The present invention thus provides the art with tools for genetically manipulating an agriculturally important pathogen. It also provides useful mutant strains which can be used effectively to reduce morbidity among ruminants, such as cattle, sheep, and goats, due to Pasteurella haemolytica.